Method of recording and reproducing stereophonic sound vibrations



1943- M. J. c. VAN DER MEULEN 2,335,275

METHOD OFYRECORDING AND REPRODUCING STEREOPHONIC scum) VIBRATIONS FiledMay 29, 1941 vvvvv VWvv V 11111 61 az-j lealen;

i770 IPA/5y Patented Dec. 7, 1943 UNITED -STATE s PATENT OFFICE METHODor RECORDING AND maraonuc- ING s'rnnnornomc soUNnvmaA'rIoNs MariusJohannes Cuinirus van der Meulen, Eindhoven, Netherlands; vested in theAlien Property Custodian Application May 29, 1941, Serial No. 395,892

In the Netherlands May 8, 1940 a g I 8 Claims. This invention relates to-a methodof recording stereophonic sound oscillations. Whenstereophonically recording sound oscillations use' is made of severalspatially separated microphones. The eiectric oscillations produced bythe microphones arefed by separate channels to severalloud-speakersarranged at adequate relative spacings. Thus a reproductionis obtained which creates the illusion of sound perspective. Theelectric oscillations derived from the microphones may be individuallyrecorded on separated sound carriers and subsequently reproduced.However, in such arrangements 'difficulty is encountered in maintainingperfect synchronlsmbetween the stereophonically associated soundrecordsduring reproduction. The least relative displacement of the soundtracks results in disturbance of the illusion of sound perspective.v

In order to maintain synchronism it has been suggested that the soundoscillations, received from the various channels should be recorded in asingle sound track so that synchronism cannot be lost. One of the knownmethods consists in modulating several carrier waves by the sound of thevarious channels and in recording the carlations of both channels arecaused to modulate material may be subjected the two sides of the soundtrack are always located opposite to each other.

According to the invention, the sound record thus made is reproduced byscanning the sound track by means of a single photo-electric cell,

splitting-up of the positive and negative halves of the carrier wave bymeans of a device known per se, demodulating each half, filtering-outthe carrier wave and reproducing each of the two channels thus obtainedby separated reproducing devices. a

required forfiltering-out the carrier wave, these rier waves thusmodulated jointly in one track.

In reproduction these carrier waves have to be separated by filters, thevarious sound frequencies being then again rendered audible by demodula-A further well known method consists in the use of two channels one ofwhich is recorded in an unchanged manner, whereas the other is caused tomodulate a carrier wave lying beyond the frequency range of the firstchannel. The modulated carrier wave is then recorded in the same soundtrack but since its frequency range lies beyond that of the firstchannel it can be reproduced separately with the aid of a filter.

All of the. above-mentioned known methods, however,'have thedisadvantage that the reproducing system is rather involved. It isnecessary to provide filters having totransmit a given sharply definedfrequency band and entirely out of! the frequency lying outside. Suchfilters are highly involved whilst their operation is not final so thatby-sounds are always audible during reproduction.

The invention has for its. object to make a stereophonic sound record intwo channels in a single sound track, it being possible for this soundrecord to be reproduced by a comparatively simple equipment.

According to the invention, the sound oscilfilters ri'eed only to filterout a single given fre-..

quency. 1 Such filters are of very simply construction and theiroperation may be fairly complete. The highest sound frequency that canbe transmitted by the method according to the invention is equal to halfthe carrier-wave frequency used and hence when using a carrier frequencyof 7000 cycles/sec. the maximum sound frequency is 3500 cycles/sec. Theinvention is also based on recognition of the fact that U suchvivification of the sound reproduced oc-- curs that the above-mentionedlimitation of the frequency. band to be reproduced has no disturbingeiIect.

' In one embodiment of the invention a further improvement in soundtransmission is enabled by the use of a recording method known per soaccording to which the sound track exhibits width and depth variationscut through an opaque covering layer of a carrier by means of a V-shapedcutter having an apical angle of about 174. This recording method evenpermits of faithfully recording frequencies above 7000 cycles/sec. It isthus possible to obtain a record up to 9000 cycles/sec. and this permitsof the range of the soundfrequencies to be record- 'ed being increasedto. 4500 cycles/sec.

In order that the invention may be clearly understood and readilycarried into efiect it will Although on the reproducing side filters areReferring to Fig. 1, l and 2. designate two recording microphones whichjointly with the associated microphone amplifiers 3 and 4 constitute twochannels 25 and 26 by which the sound I oscillations are transmittedwhile conserving the stereophonic effect. Before the recording deviceare connected two electric discharge tubes 29 and 30 which are coupledto both recording conductors to the loudspeakers l6 and 2| inchannels bymeans of a suitable transformer 21. This transformer 21 has fed to it acarrier wave by means of an oscillator device 28 which generates afrequency of 7000 cycles/sec. or higher. The value of the amplitude ofthis carrier wave and the characteristics of the tubes 29 and 30 aresuch that these tubes operate in the curved of their anode-gridcharacteristic curve so that the sound oscillations supplied by thechannels 25 and 26 are caused to modulatethis carrier wave. Thismodulation occurs in such a manner that the amplitude peaks of a givendirection, for example the positive direction, are modulated by thesound frequencies from the channel 25 and the amplitude peaks of theopposite the interposition of a transformer 40 and a recording amplifierI, to a recording device 5 which records a sound track on a carrier.This recording device may be of any known construction, 1. e. it may bea photographic recorder or a mechanical recorder, so long as at least afrequency of 7000 cycles/sec. can be recorded thereby.

' Fig. 3 shows a device .by means of which a sound record recorded inthe manner shown in Fig.- ican be reproduced. The sound track is scannedby means of asingle photo-electric cell II. The ,electric oscillationsthereby generated are amplified in a photo-electric cell amplifier 32and are fed to two oppositely connected valves =41 and 42. A separationof the carrier wave into I two. opposite parts is brought about by thesevalves, one of whichsolely' transmits the positive amplitudes, whereasthe other valve solely transmits the negative amplitudes. In thesevalves, which are constructed as rectifiers, also demodu- The.

assaave sired, this aggregate may be assembled in an appliance, thuspermitting the system to be rebuilt for stereophonic reproduction by theaddi-' tion of'the said appliance and a second loudspeaker.

Since, as is well-known, the stereophonic effect is practicallyimperceptible in the reproduction of sound oscillations below 300cycles/sec.

it is suflicient to feed these low tones in only one of the tworeproducing channels tothe loudspeaker for the reproduction of thelow-frequencies. This may be effected by supplying a third loudspeaker,which is arranged between the loudspeakers l6 and 2| with theinterposition of a filter that cuts 01? the frequencies above 300cycles/sec. In such anarranger'nent the supply clude filters whichsuppress low 300 cycles/sec.

What I claim is: 1. A method of stereophonically reproducing soundoscillations derived from two spatiallyseparated sound-receivingelements, comprising the steps of generating a carrier wave of at leastabout 7000 cycles per second, modulating only the oscillations bethepositive amplitude peaks of said carrier wave Q with the oscillationsderived from one of said elements, modulating only the negativeamplitude peaks of said carrier wave with the oscillations derived fromthe other .of said elements, combining the modulated positive andnegative amplitude peaks of the carrier wave,

recording the said modulated carrier wave to form a single sound trackon a sound carrier, scanning said sound track and reproducing the saidcarrier wave, separating the negative and positive amplitude peaksthereof into individual channels and converting the oscillations in eachof said channels into sound waves by means of separate sound reproducingdevices.

form a single modulated carrier wave, and recording. the said modulatedcarrier wave to form a single sound track on a sound carrier.

3. A method of stereophonically recording on a 1 sound carrier soundoscillations derived from two lation occurs by rectification. Each ofthe halves ofthe carrier wave which is modulated by the 5 "soundoscillations received from one'of the two channels is amplified intheintermediary ainplifler 43, M. .The sound ,frequency still pres-.-

be readily arranged in an existing system in replacement of anon-stereophonic reproducing de- .vice. For this purpose it is necessaryto provide two separated reproducing devices. Gen- :erally, twoamplifiers are already provided so that the interconnection of thevalves 4| and 42 and of the filters .36 and 31 suilices. IIdespatially-separated sound-receiving elements, comprising the steps ofgenerating a carrier wave of at least about 7000 cycles per second.modulati'ng only the positive amplitude peaks of said carrier wave withthe oscillations derived from I a sound track having both width anddepth variations.

4. A method of stereophonically reproducing sound oscillations derivedfrom two spatiallyseparated sound-receiving elements, comprising thesteps of generating a carrier wave of at least about 7000 cycles persecond, modulating only the assasve positive amplitude peaks oi'saidcarrier wave with the oscillations derived from one of said elements,

modulating only the negative amplitude peaks of said carrier wave withthe oscillations derived from the other of said elements, combining themodulated positive and negative amplitude peaks of the carrier wave,recording the said modulated carrier wave to form a single sound trackon a sound carrier, scanning the sound track by means of a photoelectriccell and reproducing the modulated carrier wave, rectifying the carrierwave and separating the negative and positive peaks thereof intoindividual channels, suppressing the carrier wave in each channel andseparately repro-. ducing the oscillations in each of said channels.

5. A method of stereophonically reproducing sound oscillations derivedfrom two spatiallyseparated sound-receiving elements, comprising thesteps of generating a-carrier wave of at least about 7000 cycles persecond modulating only the positive amplitude peaks of said carrier wavewith the oscillations derived from oneof said elements,

modulating only the negative amplitude peaks of said carrier wave withthe oscillations derived from the other of said elements, combining themodulated positive and negative amplitude peaks of the carrier wave,recording the said modulated carrier wave to form a single sound trackon a sound carrier, scanning said sound track and reproducing the saidcarrier wave, separating the negative and positive peaks thereof intoindividual channels, converting the oscillations having frequenciesgreater than about 300 cycles per second in each of said channels intosound waves by means of separate sound reproducing means,

and converting the oscillations having frequencies less than about 300cycles per second into sound waves by means of a single reproducingdevice actuated by only one-half of the modulated carrier wave. p

6. A device for recording stereophonic sound oscillations, comprising aplurality of sound-receiving elements. a plurality of electron dischargetubes having input and output electrodes and each associated with one orsaid elements by means of its input electrode. meansto generate acarrier wave of at least about 7000 cycles per second, means to applysaid carrier wave in phase opposition to the input electrodes of saiddischarge tubes and to render said discharge tubes alternatelyconductive; means to combine the outputs of'said discharge tubes and asingle recording device actuated by said combined outputs.

7. A device for reproducing a sound track comprising a recorded carrierwave having positive amplitude peaks modulated in accordance withoscillations derived from one of a plurality of spatially-separatedsound-receiving elements and filter in each of said channels forsuppressing the carrier wave component ofsaid electrical oscillations,and means for converting the modulation component of said electricaloscillations in said channels into'sound waves. V

8. A sound carrier comprising a recorded carrier wave having positiveamplitude peak variations conforming to oscillations derived from one ofa plurality of spatially-separated soundreceiving elements and havingnegative amplitude peak variations conforming to oscillations derivedfrom another of the plurality of spatially-separated soun d -receivingelements.

MARIUS (JOHANNESZ oummus van on MEULEN.

